/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode head = ListNode(-1);
        ListNode *p = &head;
        
        while( l1 && l2 ){
            if( l1->val < l2->val ){
                p->next = l1;
                l1 = l1->next;
            }else{
                p->next = l2;
                l2 = l2->next;
            }
            p = p->next;
        }
        
        p->next = (l1)? l1 : l2;
        return head.next;
    }
};
